Raman Spectroscopy
► The Raman effect is based on inelastic scattering of a monochromatic incident radiation - routine energy range: 200 - 4000 cm–1 - comprises a very small fraction, about 1/107 of the incident photons.
virtual
state
Scattered
Excitation v” = 1
v” = 0 Infrared Raman (absorption) (scattering) ► Great for many samples! - minimal sample preparation (gas, liquid, solid) - compatible with wet samples and normal ambient - sample fluorescence is problematic!! The history of Raman effect
► Discovered in 1928 by Sir Chandrasekhra Venkata Raman, using sunlight as a source, telescope as a collector, his eyes as a detector. ► Subsequent mercury sources replaced by lasers in 1962 ► Photographic plates replaced by photomultiplier tubes by 1953 ► Double and triple monochromators introduced in 1960s; holographic gratings in 1968 ► FT-Raman, array detectors, Raman microscopes more recent
► Sir Chandrasekhara Venkata Raman was the first scientist to describe and explain in the review Nature, in 1928 March 31, the experimental observation of this phenomenon in liquids.
It must be recalled that the theoretical principle of this effect was predicted in 1923 by A. Smekal, an Austrian physicist. Other research groups was working on similar subjects at the same time (during 1928):
- L. Mandelstam, G. Landsberg in URSS (May 6) - Jean Cabannes in France (ending of 1928). C. V. Raman and K. S. Krishnan, Nature, 121 (3048), 501, March 31, 1928
If we assume that the X-ray scattering of the 'unmodified' type observed by Prof. Compton corresponds to the normal or average state of the atoms and molecules, while the 'modified' scattering of altered wave-length corresponds to their fluctuations from that state, it would follow that we should expect also in the case of ordinary light two types of scattering, one determined by the normal optical properties of the atoms or molecules, and another representing the effect of their fluctuations from their normal state. It accordingly becomes necessary to test whether this is actually the case. The experiments we have made have confirmed this anticipation, and shown that in every case in which light is scattered by the molecules in dust-free liquids or gases, the diffuse radiation of the ordinary kind, having the same wave- length as the incident beam, is accompanied by a modified scattered radiation of degraded frequency.
The new type of light scattering discovered by us naturally requires very powerful illumination for its observation. In our experiments, a beam of sunlight was converged successively by a telescope objective of 18 cm aperture and 230 cm focal length, and by a second lens was placed the scattering material, which is either a liquid (carefully purified by repeated distillation in vacuum) or its dust-free vapor. To detect the presence of a modified scattered radiation, the method of complementary light-filters was used. A blue-violet filter, when coupled with a yellow-green filter and placed in the incident light, completely extinguished the track of the light through the liquid or vapor. The reappearance of the track when the yellow filter is transferred to a place between it and the observer's eye is proof of the existence of a modified scattered radiation. Spectroscopic confirmation is also available.
Some sixty different common liquids have been examined in this way, and every one of them showed the effect in greater or less degree. That the effect is a true scattering, and secondly by its polarization, which is in many cases quite strong and comparable with the polarization of the ordinary scattering. The investigation is naturally much more difficult in the case of gases and vapors, owing to the excessive feebleness of the effect. Nevertheless, when the vapor is of sufficient density, for example with ether or amylene, the modified scattering is readily demonstrable. F1 F2 C S “dark”
C F1 F2 S “light”
Hg lamp C6H6 Raman spectrum 1928 ► Discovered the inelastic Chandrasekhara Venkata Raman (1888-1970) scattering phenomenon 1930 ► The Nobel Prize for Physics Raman scattering: polarizability of molecule changes during vibration!
► Polarizability is related to how easily the molecular orbital of a molecule can be deformed ►An electric field will distort the molecular orbital (electron cloud) ►This is a weak effect that grows with the square of the electric field intensity
The electric field can distort the electron cloud of a molecule, creating an “induced” electric dipole moment
mind = aE Polarizability (a) The induced electric dipole moment ► The oscillating electric field of the incident electromagnetic radiation will create an oscillating induced electric dipole moment which will emit electromagnetic radiation (scattered radiation) ►Rayleigh scattering: sc ≈ ex elastic interaction;
(no transfer of energy between molecule and photon)
Stokes ► Raman scattering: ≠ inelastic interaction; sc ex Anti-Stokes (transfer of energy between molecule and photon)
Stokes lines: energy of photon decreases: Vibrational Raman selection rules: sc < ex (vibrational energy of molecule increases)
Anti-stokes lines: energy of photon increases: ► Polarizability of molecule must change during a particular vibration sc > ex (vibrational energy of molecule decreases) (gross selection rule) ► Δv = ± 1 (vibrational quantum = + or = - AS ex vb S ex vb number/vibrational energy levels) (specific selection rule) Classical theory of Raman scattering
(Fourier series) Classical theory correctly predicts that Raman scattering should be weaker than Rayleigh scattering and that there is a simple linear dependence of Raman scattering on incident intensity and on sample concentration.
• Expression for induced dipole moment can be written in terms of Cartesian components.
• For μ we can use a matrix equation. (μ and E are vectors, α is a tensor)
• Polarizability tensor is usually symmetric
•Polarizability is divided into isotropic (s) and anisotropic (a) parts: α = αs + αa a’ = a/Q (the polarizability derivative)
Boltzmann distribution is a major factor in determining relative Stokes and anti- Stokes intensity. The excited vibrational state will be only thermally populated, and Stokes intensity will be much larger than anti-Stokes.
4 4 h vib hc k I Stokes 0 vib kT 0 k kT e e I antiStokes 0 vib 0 k Rayleigh Stokes Anti-Stokes Raman intensity is proportional with Raman scattering cross-section ( [cm2]) Raman scattering cross-sections = target area presented by a molecule for scattering – depend on excitation frequency
- the scattering solid angle
2 -28 2 Process Cross-Section of (cm ) lex (nm) (x10 cm ) absorption UV 10-18 532.0 0.66 -21 absorption IR 10 435.7 1.66 -19 emission Fluorescence 10 368.9 3.76 scattering Rayleigh 10-26 355.0 4.36 scattering Raman 10-29 319.9 7.56 scattering RR 10-24 282.4 13.06 scattering SERRS 10-15 CHCl : C-Cl stretch at 666 cm-1 scattering SERS 10-16 3
Aroca, Surface Enhanced Vibrational Spectroscopy, 2006 Stokes vs Anti-Stokes
Rayleight ν = ν Ray ex Ray ex 1 Stokes νStokes = νex- νvib S ex vib ex lex Anti-Stokes νAntiStokes = νex+ νvib AS ex vib
CCl4 Stokes:
S ex S vib
Anti-Stokes:
AS ex AS vib
The intensity of Stokes bands is not temperature dependent. The intensity of Anti-Stokes bands is temperature dependent. A vibration is Raman active if the polarizability of molecule changes during vibration: a 0 Q k 0
The value of first derivative of polarizability at the origin (tangent, slope) influence the intensity of Raman band.
For small molecule, polarizability can be figured like an ellipsoid:
Symmetric stretching vibration of CO2 Asymmetric stretching vibration of CO2
► Polarizability changes during ► Polarizability does not change during vibration vibration → Raman band at 1340 cm-1 → no Raman band near 2350 cm-1 ►Dipole moment does not ► Dipole moment does change → no IR absorption band at 1340 cm-1 → IR absorption band at 2349 cm-1 1 vibration is Raman active (the ellipsoid size is changing).
2 and 3 vibrations are Raman inactive (the variations of size and shape are only apparent!) - in extreme positions the ellipsoid has the same shape and size therefore, the function (da/dQ)0 slope at the origin is zero. ν1 vibration - Raman active (the ellipsoid size is changing).
ν2 vibration - Raman active (the ellipsoid shape is changing).
ν3 vibration - Raman active (the ellipsoid orientation is changing).
The symmetric stretch (ν1) determine a very strong Raman band, while asymetric vibration (ν3) are very weak.
In practice is accepted the idea that Raman spectrum of H2O molecule has a single band at 3652 cm-1!!! (due to symmetric stretching) Depolarization ratio
The Raman scattered light is emitted by the stimulation of the electric field of the incident light. Therefore, the direction of the vibration of the electric field (polarization direction) of the scattered light might be expected to be the same as that of the incident light. In reality, however, some fraction of the Raman scattered light has a polarization direction that is perpendicular to that of the incident light. This component is called the perpendicular component. The component of the Raman scattered light whose polarization direction is parallel to that of the incident light is called the parallel component, and the Raman scattered light consists of the parallel component and the perpendicular component.
The depolarization ratio (in Raman spectroscopy) is the intensity ratio between I the perpendicular component and the parallel ρ component of the Raman scattered light I|| 2 4 I m mx=axyEy my=ayyEy 2 I Ix αxy ρ 2 I|| Iy αyy 3γ2 ρ Rayleigh: 2 45α 4γ2 1 α (α α α ) 3 xx yy zz 1 γ2 [(α α )2 (α α )2 (α α )2 6(α2 α2 α2 ) 2 xx yy yy zz zz xx xy yz xz
α is non-zero only for totally symmetric vibrations. For totally symmetric vibrations 0 ≤ ρ ≤ 3/4 For non-totally symmetric vibrations ρ = 3/4 3γ2 Raman: ρ (Polarised laser radiation: ρmax= 3/4) 45α' 4γ2 a α α' ' If laser radiation is unpolarised: Qk Q k ρmax= 6/7. ► ρ = 0 total symmetric vibration 6'2 ► 0 < ρ < 3/4 partial symmetric vibration ρ 2 2 ► ρ = 3/4 = ρmax total asymmetric vibration 45α' 7γ The value of the depolarization ratio of a Raman band depends on the symmetry of the molecule and on the symmetry of normal vibrational mode.
ρ = 0 total symmetric vibrational mode (cubic point grups) 0 < ρ < 0.75 partial symmetric vibrational mode ρ = 0.75 total asymmetric vibrational mode
ρ < 0.75 polarized band 3γ2 ρ = 0.75 depolarized band ρ 45α' 4γ2 Raman Scattering vs IR Absorption Raman Scattering ► involves changes in the polarizability (α) of the molecule (induced dipole) Raman spectroscopy ► the electric field of the molecule oscillates at the frequency of the incident wave (the induced dipole emits electromagnetic radiation!) ► if induced dipole is not constant, inelastic scattering is allowed (Raman) ► if induced dipole is constant, scattering is elastic (Rayleigh/Mie)
IR absorption IR spectroscopy - involves changes in the dipole moment (µ) of the molecule (during vibrations) - - some vibrational modes are both IR and Raman active!
Infrared and Raman spectroscopy are two kinds of spectroscopy - the spectrum is a graph of light intensity as a function of light frequency - peaks in the spectrum give information about molecular structure - from molecular structure, the compound can be identified The rule of “mutual exclusion” If a molecule has a center of symmetry, then no transition is allowed in both its Raman scattering and IR absorption, but only in one or the other.
CO2: Band Infrared Raman -1 ν1 - symmetric stretching (1330 cm ) inactive active -1 ν2 - asymmetric stretching (2349 cm ) active inactive -1 ν3 - bending (667 cm ) active inactive
For CO2, vibrational modes that are IR active are Raman inactive and vice-versa.
The general rule: symmetric vibrations give intense Raman bands and weak IR bands.
The fact that H2O does not obey the rule of mutual exclusion indicates that the H2O molecule is not centrosymmetric (it is bent).
As expected, the ν1 symmetric stretch is also strongly Raman active.
H2O: Band Infrared Raman -1 ν1 - symmetric stretching (3652cm ) strong strong -1 ν2 - asymmetric stretching (3755cm ) very strong weak -1 ν3 - bending (1595cm ) very strong weak
Raman vs Infrared Spectra Combined Infrared and Raman: structure determination and band assignment Structure determination and band assignment rely on several factors: (i) Number of fundamental vibrational modes There are (3N – 5) vibrational modes for linear molecules and (3N – 6) vibrational modes for bent molecules. Note that if a molecule is linear, perpendicular vibrations are always doubly degenerate, and thus appear in spectra as a single bands. Hot bands are likely to be present, but are generally weaker because of population considerations. Hot bands which involve transitions between different vibrational modes may also appear. Anharmonicity leads to overtones, which are also expected to be weak compared to fundamental bands. (ii) Vibrational frequency Bending modes occur at the lowest frequencies (wavenumbers), while asymmetric stretches modes are highest. Modes above 2500 cm-1 always involve H atoms! Use group vibrational frequencies to exploit isotopic frequency shifts. (iii) Q branches The presence of Q branches in the IR indicates either a non-linear molecule, or a bending vibration in a linear molecule. To establish which, look for the stretching modes: if these have no Q branch the molecule is linear. (iv) Rule of mutual exclusion This indicates whether the molecule is centrosymmetric (possesses a centre of inversion). (v) Vibrational dipole moments and polarizability Whether a particular vibration generates a large dipole change, or a significant change to the polarizability can be related qualitatively to the strengths of the IR or Raman bands.
For example the more symmetric the vibration, the more Raman active the mode (e.g. HCN 1). Examples of structure determination and band assignement
McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed., Wiley, New York: 2000 Nature of the Raman and IR processes
Raman scattering IR absorption
2-photon process (νex νscatt) 1-photon process (νincident) conserves wave function parity changes wave function parity Raman cross sections weak (10-29 cm2) (10-21 cm2)
Selection rules Raman IR rotational transitions Δ J = 0, 2 (anisotropic polarisability) Δ J = 0, 2 ( only roto-vibrational!) vibrational transitions Δ v = 1 (polarisability change) Δ v = 1 (dipol moment change)
IR vs Raman a) Complementarity for vibrational modes (Rule of mutual exclusion for centrosymmetric molecules) b) Experimental considerations (Raman): i) Wavelength of primary radiation can be conveniently chosen ii) Polarisation features can help with identification of nature of modes iii) Resonance Raman: stronger signal (and potentially selective) iv) Raman can be applied with good spatial resolution v) Raman scattering by water is weak (compared to IR) vi) Raman has an essentially frequency independent penetration depth vii) For samples with significant fluorescence background (excited by the primary radiation), it may be difficult to distinguish the Ramansignal from the background. Raman instrumentation
Instrumentation for modern Raman spectroscopy consists of three components: - a laser source, - a sample illumination system - a suitable spectrometer.
Source The sources used in modern Raman spectrometry are nearly always lasers because their high intensity is necessary to produce Raman scattering of sufficient intensity to be measured with a reasonable signal-to-noise ratio. Because the intensity of Raman scattering varies as the fourth power of the frequency, argon and krypton ion sources that emit in the blue and green region of the spectrum have and advantage over the other sources.
Typical lasers:
Ultra-violet: 244, 257, 325, 364 nm Visible: 457, 473, 488, 514, 532, 633, 660 nm Near Infra-red: 785, 830, 980, 1064 nm Sample Illumination System
- simpler than for IR spectroscopy (glass can be used for windows, lenses, and other optical components) .
- the laser source is easily focused on a small sample area and the emitted radiation efficiently focused on a slit.
- very small samples can be investigated.
Liquid Samples: Aqueous solutions can be studied by Raman spectroscopy (water is a weak Raman scatterer) but not by infrared! This advantage is particularly important for biological and inorganic systems and in studies dealing with water pollution problems.
Solid Samples: Raman spectra of solid samples are often acquired by filling a small cavity with the sample after it has been ground to a fine powder. Polymers can usually be examined directly with no sample pretreatment. In order to block Rayleigh scattering from reaching the detector it is use a Raman notch or Raman edge filter.
► Notch filters transmit both Stokes and anti-Stokes Raman signals while blocking the laser line.
►Edge filters (also known as barrier filters) transmit either Stokes (longpass) or anti-Stokes (shortpass).
Notch filter LWP edge filter SWP edge filter
Filters for simultaneous Filters for only Stokes Filters for only Anti- Stokes and Anti-Stokes measurements Stokes measurements measurements
An edge filter can provide a superior alternative, as they offer the narrowest transition to see Raman signals extremely close to the laser line. Raman Spectrometers
- Classical Raman spectrometers (monochomators with photomultipliers as transducers)
- FT Raman spectrometers (Fourier transform instruments equipped with cooled germanium transducers or multichannel instruments based upon charge-coupled devices).
- Micro-Raman spectrometers NON-CLASSICAL RAMAN EFFECTS
. 1 Surface-enhanced Raman effect The influence of small metal (silver, gold, copper) particles such as colloids or roughened surfaces on the elementary process of Raman scattering can enhance the intensity of the Raman effect by several orders of magnitude. This effect is used in surface-enhanced Raman spectroscopy (SERS).
2 Resonance Raman effect Resonance Raman spectroscopy (RRS) makes use of an excitation source with frequency close to a molecular electronic absorption frequency. Under these conditions a resonance occurs which may enhance the intensities of the Raman lines by several orders of magnitude, especially those connected with totally symmetric vibrations of the chromophore
Resonance and surface-enhanced Raman effects may be combined to produce surface- enhanced resonance Raman spectroscopy (SERRS).
RRS, SERS, and SERRS can be recorded with the same spectrometers as classical Raman spectra, although different conditions of the excitation and special sample techniques are used.
They are important techniques for trace chemical analyses. SERS (Surface enhanced Raman spectroscopy or surface enhanced Raman scattering) is a surface-sensitive technique that enhancesRaman scattering by molecules adsorbed on rough metal surfaces. The enhancement factor can be as much as 1010 to 1011, which means the technique may detect single molecules. Two mechanisms (still in debate!): 1)the chemical theory: the formation of charge-transfer complexes – applies only for molecule which have formed a chemical bond with the surface. 2)the electromagnetic theory : the excitation of localized surface plasmons - apply even in those cases where the molecule is only physisorbed to the surface.
Surface plasmon resonance (SPR) is the collective oscillation of valence electrons in a solid stimulated by incident light.
The resonance condition is established when the frequency of light photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei.
Surface plasmon resonance in nanometer-sized structures is called localized surface plasmon resonance (LSPR) Resonance Raman spectroscopy Resonance Raman spectroscopy uses incident radiation of frequency nearly coinciding with the frequency of electronic transitions of the sample. The intensity of scattered radiation is considerably increased in comparison with conventional Raman. Because often only a few vibrational modes contribute to the more intense scattering, the spectrum is greatly simplified.
The nine peaks in the spectrum of solid K2CrO4 are Stokes lines corresponding to the excitation of the symmetric breathing mode of the tetrahedral chromate anion and the transfer of up to nine vibrational quanta during the photon-ion collision.
The technique is used for the examination of metal ions in biological macromolecules (such as the iron in hemoglobin and cytochromes or the cobalt in vitamin B), which are present in too low abundances for conventional Raman to detect. The Raman signal can be strongly enhanced (at least 3 orders of magnitude) by exciting directly into an absorption band of the molecule. The advantage lies in the fact that by tuning the excitation wavelength, we can now selectively examine different parts of the molecule. In figure, we can see how much difference this can make when obtaining resonance Raman spectra of hemoglobin. If we use an excitation wavelength of 418 nm, this is right in the center of the absorption band for the heme group and therefore the molecular vibrations that are enhanced all arise predominately from the heme group. If an excitation wavelength of 230 nm is used, this primarily enhances the vibrations from tyrosine (Tyr) and tryptophan (Trp) amino acid residues in the protein. Different excitation wavelengths enhance Raman signal from different parts of the molecule The Raman spectrum obtained with 230 nm excitation is completely different from the one
obtained with 418 nm excitation because of the 418 nm: Excitation into heme absorption band. resonance enhancement. This gives us a great deal RR spectra from the heme of selectivity in our experiments and by choosing 230 nm: Excitation into absorption band of Tyr the excitation wavelength we can decide which part and Trp. UVRR spectra arises from those of the molecule we want to monitor. residues in the protein 3 Non-linear Raman effects If the exciting radiation has a very high intensity, then the higher terms of the polarizability expansion have to be considered:
β is the 1st molecular hyperpolarizability, and γ is the 2nd molecular hyperpolarizability, leading to non-linear Raman effects. Coherent Raman effects (two or three laser beams of different frequency):
CSRS - coherent Stokes Raman spectroscopy CARS - coherent anti-Stokes Raman spectroscopy SRS - stimulated Raman spectroscopy IRS - inverse Raman spectroscopy SRLS - stimulated Raman loss spectroscopy PARS - photoacoustic Raman spectroscopy Coherent Anti-Stokes Raman Scattering (CARS) For sample irradiated by two high-energy laser beams (P -pump and S - Stokes) (P > S) in collinear direction, beams interact coherently to produce strong scattered light of frequency (2P −S). When S is tuned to resonance: S = P - M (M = Raman active mode of thesample) a strong beam at frequency P + M is emitted: 2P − S = 2P − (P − M) = P + M CARS signal is coherent and emitted in one direction and can be observed without monochromator. It is on Anti-Stokes side and thus avoids fluorescence. All modes that are Raman active and some inactive Raman and IR modes are active in CARS.
The advantage of CARS is that it can be used to study Raman transitions in the presence of competing incoherent background radiation, and so it can be used to observe the spectra of species in flames. Spectral intensity is then interpreted in terms of the temperature of different regions of the flame. Hyper-Raman scattering is produced by very high intensity pulses. Two photons of the exciting radiation produce the Raman spectrum. A non-linear effect in which the vibrational modes interact with the second harmonic of the excitation beam. This requires very high power, but allows the observation of vibrational modes that are normally "silent".
With high intensity pulse at frequency , scattered radiation contains frequencies 2
(hyper-Rayleigh scattering) and (2 ± M) (Stokes and anti-Stokes hyper-Raman scattering),
where Μ is normal vibration of molecule, caused by two incident photons (2) of the laser.
Different symmetry selection rules apply, and hyper- Raman effect contains all IR active modes. Photoacoustic Raman Spectroscopy (PARS)
Two laser beams, P (pump beam) and S (Stokes beam) impinge on gaseous sample, interact when P - S = M for Raman active mode. The Stokes beam is amplified and the pump beam is attenuated. The vibrationally excited molecules lose excitation to translational energy, changing the pressure in the cell, which can be detected with a microphone. In this case there is no Rayleigh line, and low energy rotational lines can be studied.
For each Stokes photon created by the Raman process, one molecule is transferred from the lower state to the upper state of the transition. Collisional relaxation of these excited molecule produces changes that is detected by a microphone. Problems
1. Consider the vibrational mode that corresponds to the uniform expansion of the benzene ring. a) Is it Raman active or infrared active? b) Why?
2. A linear molecule has the following fundamental vibrational transitions: 370 cm-1, 520 cm-1, 880 cm-1 and 1550 cm-1 (all Raman active) The wavelength of the excitation radiation is 500 nm. a) Calculate the wavenumbers of scattered radiation in Stokes range. b) How many atoms has this molecule?
3. Calculate the temperature of the sample (CCl4) from Raman spectrum, knowing -1 the excitation wavelength and IS/IAS ratio for 460 cm band (IS/IA= 6; lex = 488 nm)
-34 8 -23 h = 6,63∙10 J∙s, c= 3∙10 m/s, kB=1,38∙10 J/K